Caution: PKT files execute code. Downloading from untrusted sources can crash Civil 3D or, in extreme cases, introduce macro viruses. Stick to these verified sources:
A subassembly pkt file is a package file that contains one or more subassemblies. These files are used to distribute and install subassemblies in Civil 3D. When you download a subassembly pkt file, you are essentially downloading a package that contains one or more subassemblies that can be used in your designs. Civil 3d Subassembly Pkt- Download